Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures

Appendix H Evaluation of the Potential for Further Expansion due to ASR (Prognosis)

Test Procedure for Determination of the Water-Soluble Alkali Content of Concrete

H.1 Introductory Remarks

H.1.1 Alkali concentration in the pore solution and effect of ASR

The OH-ions are the aggressive species towards the aggregates; however, they are supplied by the portlandite Ca(OH)2 to reach about equilibrium with the alkali cations in the pore solution. Consequently, the higher the alkali concentration in the pore solution, the higher the [OH-] (then the pH), and the higher the risk for AAR. In the absence of supplementary cementing materials, the higher the cement alkali content and the lower the water-to-cement ratio (lower amount of pore water), the higher the alkali and OH-concentrations in the pore solution.

In the presence of ASR, reaction products are formed that contain silica, calcium, and alkalis, which were originally present in the pore solution. As ASR progresses and more reaction products are formed, the alkali concentration in solution progressively decreases, reducing the [OH-] and the intensity of ASR. Some experiments suggest that ASR expansion in the presence of highly reactive natural aggregates can be minimized when the alkali concentration in the pore solution falls under 0.6N [NaOH+KOH] in the long term (Duchesne and Bérubé 1994a). Therefore, the precise determination of this concentration could be useful for determining whether or not this concentration is sufficient for ASR be maintained, and may facilitate decisions on the measures of mitigation/remediation.

H.1.2 Water-soluble alkali content of concrete

It is very difficult to extract pore solution from old concrete, which could then be chemically analyzed for alkali and OH-concentrations (Bérubé and Tremblay 2004). A more common but indirect method consists in determining the active- or water-soluble alkali content of concrete, on a kg/m3(or lb/yd3) Na2Oeq basis, by subjecting a representative ground sample of the concrete under study to the hot-water extraction method first proposed by Rogers and Hooton (1993). These alkalis were present in the concrete pore solution before the sample was dried before testing. The alkali and OH-concentrations in the original pore solution could then be calculated from the water-soluble alkali content (see Section H.3). Bérubé et al. (2002a) applied the hot-water extraction method to a number of concretes incorporating various types of aggregates showing different degrees of alkali-silica and alkali-carbonate reactivity, thus allowing determining the potential alkali contribution by the aggregates themselves during the test. The procedure described in this appendix is based on the results and the recommendations of this study.

H.1.3 Distinction between total and water-soluble alkalies

Limiting the total-alkali content in concrete is considered to be an effective way of minimizing concrete expansion in the presence of aggregates susceptible to ASR (see Report on Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction, Thomas, et al. 2008). However, a clear distinction must be made between total alkalis (often restricted to the total amount of alkalis in the cement) and water-soluble alkalis (as measured in the hot-water extraction method). In fact, a significant amount of the cement alkalis are incorporated in the cement hydrates and, in the presence of ASR, in the products from this reaction. Previous studies (Taylor 1990, Duchesne and Bérubé 1994b) indicate that hydrates from ordinary portland cement (OPC) contain about to 0.5 percent Na2Oeq. The results by Diamond (1989) and Duchesne and Bérubé (1994b) also suggest that the alkali content in hydrates from OPC is about proportional to the total amount of alkalis contained in the cement used. Based on these findings, one considers that about 40 percent of the total alkalis from the cement are incorporated in hydrates from OPC concrete. In such a situation, 3 kg/m3 (5lb/yd3) of total Na2Oeq, for instance, would correspond to about kg/m3 (3 lb/yd3) of water-soluble Na2Oeq (i.e., 60 percent x 3 kg/m3) (60 percent x 5 lb/yd3).

H.1.4 Usefulness and limitations of the hot water extraction method

The water-soluble-alkali content of concrete is a useful parameter for determining whether or not the concrete tested contains sufficient alkalis for ASR to develop (diagnosis) or to be sustained (prognosis). In the approach proposed in the Appendix I the water-soluble alkali content is used, in combination with other relevant information (e.g., expansion tests on cores, environmental and stress conditions in the field), to estimate the current expansion rate in ASR-affected concrete structure; the higher the water-soluble-alkali content, the higher the value calculated for the current expansion rate.

Nevertheless, the experimental variability of the test method is relatively high, the estimated coefficient of variation ranging between 10 and 15 percent, even when using a control concrete (Bérubé et al. 2002a).

H.2 Test Procedure

H.2.1 Sampling of concrete and storage before testing

The water-soluble alkali content may vary considerably within a given structure and even within a single member (Bérubé et al. 2002b). This may be due to variations in the concrete mixture proportioning, the extent of ASR (which consumes alkalis) and cracking (alkali concentration through evaporation or alkali leaching along cracks), and in the exposure conditions which may greatly affect the water-soluble alkali content in the near-surface concrete (external sources of alkali, alkali migration in/out of the sampled zones, alkali concentration at the surface through evaporation, or alkali leaching by rain). It is then recommended that the test method be applied to concrete samples, usually cores, taken in different members exposed to various conditions. Moreover, the concrete close to the surface should be discarded or analyzed separately, as this concrete is more susceptible to contamination by external sources of alkalis, or alkali leaching by rain, particularly if it is cracked or very porous, or to alkali concentration through evaporation.

The concrete samples should be wrapped with plastic film/sheets and placed in sealed plastic bags immediately after coring, to prevent any drying (or wetting).

H.2.2 Preparation of concrete samples

A representative sample of minimum 2 kg (4.5 lbs) of concrete should be crushed with a hammer to < 25 mm (1 in) and allowed to dry immediately before testing. Density and water content measurements could be performed at the same time to further estimate the alkali concentration in the concrete pore solution (in mole/liter of NaOH+KOH), to which the extent of the reaction is more directly related (see Section H.3).

After drying, the concrete sample is progressively crushed and ground to pass the no. 100 sieve (< 160 µm).21Two 10-g (0.02 lbs) representative sub-samples are taken for the test.

Two 10-g representative sub-samples of a control concrete with a well-known water-soluble alkali content are also prepared and tested in parallel.22,23

H.2.3 Hot-water extraction

Each 10-g (0.02 lbs) sub-sample of the concrete under study and the control concrete are individually immersed in 100 mL of distilled water. The distilled water is boiled for 10 minutes and the suspension is allowed to stand overnight at room temperature. The next morning, the suspensions are filtered and the solution volume topped to 100 mL by adding distilled water.

H.2.4 Chemical analyses and expression of results

The sodium and potassium concentrations in the solution are determined by flame photometry or atomic absorption in accordance with Clause 17.1 ("Total Alkalis") of ASTM C 114. The results are expressed in %Na2O, %K2O, and %Na2O equivalent per kg of concrete and averaged for each concrete tested (sample and control).24The average result for each concrete, in %Na2Oeq, is expressed in kilograms (lbs) of Na2O equivalent per cubic meter (yd3) of concrete, provided the corresponding concrete density (ω, in kg/m3or lb/yd3) is known or reasonably estimated25:

H.2.5 Correction with respect to the control sample

The (average) result obtained for the concrete under study is normalized by comparison with that obtained for the control concrete (known water-soluble content), before applying a correction for the contribution by aggregates described in Section H.2.626:

H.2.6 Correction for the contribution by aggregates in the test

The result obtained above is adjusted for the alkali contribution by aggregates during extraction, which greatly varies from one aggregate to another.27The water-soluble alkali content measured for a number of different rock types from Canada, are given in Table H1. In absence of any information, reasonable estimates may be obtained from the values given in Table H1.

1 Based on aggregate factors of 1,050 kg/m3 for the coarse aggregates and 700 kg/m3 for the granitic sand.

2 Contribution by the corresponding coarse aggregate (1,050 kg/m3) plus the one by the granitic sand (700 kg/m3).

H.2.7 Interpretation of results

Since about 40 percent of the cement alkalis are incorporated in the cement hydrates in the case of OPC concrete (see Section H.1.3), the (measured) amount of water-soluble alkalis is generally lower than the (calculated) amount of total alkalis from the cement (calculated as the cement factor times the cement alkali content). However, some alkalis may be supplied to the pore solution by other concrete constituents (aggregates, mineral admixtures, mixture water, superplastisizer etc.) and external sources (deicing/anti-icing chemicals, sea water etc.). At the same time, alkalis may be progressively incorporated in the ASR products, migrate in/out of the sampled concrete zones, concentrate by evaporation near the concrete surface, and/or be leached from this surface or along cracks by rain or running water. Most of the above mechanisms will be most active in the outer portions of the concrete members and their effect could thus be minimized by taking concrete samples at sufficient depths inside the concrete members under study, as specified in Section H.2.1. The results should also be interpreted in accordance with the other limitations mentioned in Section H.1.3.

H.3 Alkali Concentration in the Concrete Pore Solution

A realistic estimate of the alkali and OH-concentrations in the concrete pore solution can be obtained if the original water content (W) of the concrete is known. This can be determined by drying, under vacuum, a representative concrete sample in an oven at 80°C (176°F) until equilibrium, provided that the concrete was not allowed to dry or to absorb humidity since coring (see Section H.2.2). The following relationship is used for calculation:

where 32.3 is a constant to transform kg/m3 Na2Oeq mass units to moles/liter [Na+K] concentration units, ω28 is the concrete density in kg/m3 (see Note 1) and W is the pore water content of the concrete (in %), as determined by mass loss after drying until equilibrium.

Bérubé, M.A., and Tremblay, H., "Chemistry of Pore Solution Expressed under High Pressure - Influence of Various Parameters and Comparison with the Hot-water Extraction Method," Proceedings of the 12thInternational Conference on AAR, Beijing (China), pp. 833-842, 2004.

21Grinding to <160 rather than <80 µm in the original procedure (Rogers and Hooton 1993) is easier, allows a lower alkali contribution by aggregates, while not significantly affecting the extraction of alkalis in solution before the concrete has dried (Bérubé et al. 2002a).

22Testing in parallel a control concrete with a well-known water-soluble alkali content on the basis of which the results are corrected (see Section H.2.5), greatly improves the precision of the test (Bérubé et al. 2002a).

23A given ground concrete can be used for control purposes. In order to prevent variations over time, the control concrete sample shall be kept sealed in a plastic bag under vacuum. Despite this, the water-soluble alkali content measurements of control specimens kept as described above were found to decrease slightly with time (Bérubé et al. 2002a); it is thus recommended that a new control sample be ground every 6 months from a representative uncrushed sample of the control concrete. In order to avoid variations in the water-soluble alkali content of the control concrete (from further hydration or ASR), the latter shall consist in a well-hydrated (e.g., kept for at least 90 days in air at >95 percent R.H. and 23°C (73°F) before the first "control" measurement is taken) non-reactive concrete which is wrapped humid in a cling film and stored in a freezer inside an air-tight plastic bag.

24For a given sample, the two results should not differ by more than 5 percent of the average.

25In absence of any information, a default value of 2,400 kg/m3(4,045 lb/yd3) can be used for the concrete density.

26For instance, if the result obtained for the control concrete is 13 percent higher than the actual (well-known) value, the result obtained for the concrete under study is automatically reduced by 13 percent using the above relationship.

27Aggregates in concrete may release significant amounts of alkalis in the test, e.g. 0.70 kg/m3 (1.18 lb/yd3) Na2Oeq on average for the 17 aggregates tested by Bérubé et al. (2002a); consequently, it is important that the results obtained be corrected for this contribution. A general trend is that the higher the total-alkali content in the aggregates, the higher the absolute amount of alkalis released in the test. The most susceptible rock types are those rich in nepheline (phonolite, nepheline syenite), feldspars (granite, granitic gneisses, rhyolitic tuff, andesite, lithic grave etc.), and clay minerals (shales, clayey limestone etc.). The aggregates with the highest contributions in the test procedure are likely those also supplying, with time, the highest amount of alkalis to the pore solution of the concrete during its service life (Bérubé et al. 2002b, Bérubé and Fournier 2004).

28In absence of any information, a default value of 2,400 kg/m3 (4,045 lb/yd3) can be used for the concrete density.